Mass flow measuring device for a gaseous medium



Feb. 24, 1970 E. M. MOFFATT MASS FLOW MEASURING DEVICE FOR A GASEOUS MEDIUM Filed March 4, 1968 /1,

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United States Patent 3,496,771 MASS FLOW MEASURING DEVICE FOR A GASEOUS MEDIUM Elbert Marston Motratt, Glastonbury, Conn., assignor to United Aircraft Corporation, East Hartford, Conn., a

corporation of Delaware Filed Mar. 4, 1968, Ser. No. 710,278 Int. Cl. G01p 5/00 U.S. Cl. 73-194 6 Claims ABSTRACT OF THE DISCLOSURE A device for measuring mass flow of a gaseous medium in a duct is provided wherein an easily sensed compressibility factor of the gas is automatically measured and incorporated with a flow velocity parameter to produce the mass flow indication.

Background of the invention In the field of gas pumping by large gas producing utilities it is very important to have an accurate knowledge of the mass flow of the gas being pumped through the pipe lines because the user is charged according to the B.t.u. content of the gas supplied. To date, devices for providing this information have involved a direct flow measurement which is combined with an estimate of the average density. The density must be estimated manually based upon periodic measurements. As a result, the indication of total daily gas purchase or consumption contains inaccuracies.

Summary of invention Accordingly it is an object of this invention to provide an automatic gaseous medium mass flow measuring device by automatically sensing a flow velocity parameter and an easily sensed compressibility factor of the gas.

The foregoing and other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of preferred embodiments thereof as illustrated in the accompanying drawings.

Brief description of the drawings FIGURE 1 illustrates a preferred embodiment of the automatic gaseous medium mass flow measuring device of this invention.

FIGURE 2 illustrates the operation of the devices 12, 14 and the sensor 18 shown in FIGURE 1.

Description of the preferred embodiment In FIGURE 1 a pipeline 10 contains a gaseous medium flowing in the direction as indicated and it is desred to know the mass flow of the gas therein. The mass flow measuring device utilizes two parameter-measuring devices, i.e., a sonic wave measuring device for providing a signal indicative of the flow velocity and a cylinder and piston device for providing an indication of the compressibility of the gaseous medium.

The sonic flow measuring device sends a pulse both downstream and upstream in the pipe and the time for the pulses to reach their respective receiving devices may be utilized to indicate the flow rate of the gas. The flow measuring device comprises two pairs of transmitting and receiving devices 12 and 14, separated by a distance L. The T transmitter represents a sonic pulse source which sends a pulse downstream towards the receiver R and similarly the transmitter T sends a pulse upstream to the receiver R In view of the directional flow the (T -R transmitter-receiver combination measures the time to send a pulse upstream and the T -R combination meas- 'ice ures the time to send a pulse downstream. The ditference in time that it takes for these pulses to travel through the gaseous medium is a parameter of the flow velocity. The sonic pulse producing device may be such as described in my copending application entitled Shock Pulse Generator, U.S. Ser. No. 631,009, filed Apr. 14, 1967. The receiving devices, i.e., the pickups, may be damped quartz piezoelectric crystals of the type shown and described in my copending applications entitled Acoustic Wave Sensor, U.S. Ser. No. 606,275, filed Dec. 30, 1966 and Gas Flow Measuring System, U.S. Ser. No. 654,786, filed July 20, 1967.

The density sensor comprises a piston 16 which reciprocates through a known displacement, AV in a compression chamber cylinder 18 of known volume. The cylinder 18 is connected to the pipeline through a restriction 20 and is bled to atmosphere through a smaller restriction 22. This small bleed 22 is to insure ample flow of gas through the cylinder 18 so that the mean pressure and temperature in the cylinder will be equal to the pipe pressure to within a tenth of a percent. The restriction 20 has to be sized in relation to piston speed and it must offer sufficient restriction to allow the piston pressure to follow the adia- 'batic relationship within a tenth of a per-cent. As the piston 16 reciprocates, it will pump some gas back and forth through the restriction 20 but this flow can be less than a tenth of a percent of the cylinder volume and may be considered negligible. The error of percent is used as a typical figure for precision measurement. This accuracy can be improved at the expense of retarding the speed of the response of the system by changing the relative size of the restrictions. Common one-way flow valve arrangements for obtaining this result may also be used.

At the top of the cylinder 18 is a pressure transducing device 24 which may be a piezoelectric dynamic pressure sensor. The sensor is insensitive to static pressure such as that in the pipeline 10, but produces an A.C. voltage output representing the peak-to-peak pressure variation, AP, in the cylinder as its gas is compressed by the piston 16. The dynamic pressure sensor eliminates the need for high response static pressure sensors and subtraction networks to obtain the pressure variation value, AP. The A.C. voltage is directly proportional to the quantity AP.

The stream velocity in the center of the pipe is V; and the sound velocity may be described as V The two quartz receivers, R and R are located at a distance L respectively upand downstream from one another and detect the time intervals t and t indicative of the times necessary for the sonic pulses to travel the distance L in the upand downstream directions.

Then: t =time for the sonic signal to travel upstream L ft:

t =time for the sonic signal to travel downstream to a very close approximation.

For an imperfect gas of unknown composition When power switch 32 is closed, an automatic cycle control 34 is actuated and performs the following se- V quence:

s 7 (l) A counter 38 is switched by means of a switch 36 to a COUNT-UP mode. The R output is switched to where K IS an adiabatic compressibility factor and equals START COUNT and R2 output is switched to a STOP E COUNT. AV (2) T is fired, giving a count on the counter equal to T7 the upstream travel time for the pulse.

(3) The counter 38 is switched to a COUNT-DOWN 1.e., the ncremental change is pressure AP for a percentage mode, the R1 Output is switched to STOP COUNT and the change in the volume AV/ V. The quantity 7 equals the R2 Outputis switched to START COUNT density of the gas and g is the gravity constant. Hence, 4 T2 is fired, Subtracting the time f the pulse to travel downstream from the previous count so that the (4) AP 1 1 5 time registered on the counter is now the ditferentlal I 7 time At. and substltlltlngEqllatlon41112y151ds (5) The counter reading is transferred to a standard D-A converter 40, e.g., a Foxboro 681 P-N Digital to 5 L t1t2=25 Analog Converter.

e (6) The voltage output from converter 40 is switched to a multiplier 42, e.g., an Applied Dynamics, Inc., Model solvlng for Vf yields 68 Quarter Square Multiplier, at the same time as the AP (6) AP voltage from sensor 24.

1 2)' (7) The output from multiplier 42 and a pre-set voltf T' age from a voltage source 44, corresponding to K are 7 switched to another multiplier 46, of the same type as The Weight flow, W, in pounds per second of the gas multiplier 41 through a mp6 equals (8) The reading on a meter 48 is erased and the output from multiplier 46 is switched at At to give the new value h A th d a h of W. w ere e Crohs secnon t e plpe and K1 Is This sequence can be repeated as frequently as desired i cons.tant to some for the velocity profile of the gas and accomplished automatically through the use of a timed m the plpe' cycle in cycle control 34.

Many other types of computing systems and read-outs could be used. For example, indicator 48 could be a simple voltmeter graduated in terms of W with a loading Substituting (6) and (7) yields 8 K A W: (tITtZXAP) device to hold the previous reading until a new one is 2L 7, received. Indicator 48 could also be a digital voltmeter,

the indicator could have a tape print-out or could intel 40 grate the totals automatically to give total flow over a 2 if period of time.

V If the voltage AP is converted to digital form with an is a constant K for the system for any gas, and (t t $33 gi gg z g ig g gi ig ggfi f mumphed auto and AP are easily measured variables. I d f For an imperfect gas the compressibility factor can be 1 A deg/ice for measuring mass flow of a gas in a duct measured directly by feeding a sample into a reciprocating a risin compressor chamber with a volumetric change AV (dis- 5 for producing Sonic Waves Which av 61 placement) and a mean volume (V), and measuring the Stream and downstream in the Gas flow throuoh a resulting pressure fluctuation AP with a pressure transsection of the duct ducer such asaplezoelecmc F'. (b) means for measilring the difference in the travel The use of such a compressibility factor measurmg detime of a sonic Wave moving upstream in the Se) vice in conjunction with the measured time (t t will tion and travel time of a sonic Wave moving down then yield an automatic mass flow measuring apparatus Stream in the Section gas of mlxture and unknown compyosmon by (0) means for measuring a compressibility factor of app ymg-Equatlon the gas flowing in the duct; and

Equanonscan [be Wmten' (d) multiplier means responsive to both measuring (9) W=KZ M AP means for producing a signal proportional to the mass flow of the gas.

2. A device according to claim 1 wherein the means for measuring a compressibility factor measures an adi- K1119 abatic compressibility factor and comprises:

Where K (a) a gas compressor having a compression chamber V of known volume and displacement, the chamber having an intake coupled to the duct for drawing off and is constant for any given system, depending only on gas and an exhaust; and the geometry. The two variable parameters are easily (1; means for sensing the gas pressure i h measured and account for density, temperature and other pression chamber and producing a Signal proportional factors which are normally required in mass flow measto the ak-to-peak pressure variation i th h urements. Since the formula is generally applicable to all gases, the system is particularly suited to pipeline installa- 3 A d i according to claim 2 wherein the gas tions Where gas Compositions y y from time to pressor and the means for sensing the gas pressure meastlmeure an adiabatic compressibility factor equal to FIGURE 2 lllustrates how the devices 12 and 14, and compressibility sensor 18 can give continuous automatic readout of weight flow of gas.

5 Where:

AP is the peak-to-peak pressure variation in the chamber and AV/ V is a constant for the device in which AV is the known displacement of the compressor and V is the mean volume of the chamber.

4. A device according to claim 1 wherein:

(a) the means for producing sonic waves includes shock pulse generators positioned at each end of the duct section;

(b) the means for measuring the travel times includes sonic pulse receivers positioned at each end of the section and between the pulse generators; and

(c) subtracting means are connected with the pulse receivers for producing a signal proportional to the difference in travel times of an upstream-moving pulse and a downstream-moving pulse.

5. A device according to claim 4 for continuously measuring mass flow wherein:

(a) the subtracting means includes:

(1) a digital counter having COUNT-UP and COUNT-DOWN modes associated respectively with the upstream-moving pulse and downstream-moving pulse; and

(2) a digital-to-analog converter receiving the digital count; and

(b) the multiplier means includes a multiplier circuit having an input from the digital-to-analog converter proportional to the digital count another input from the means for measuring the compressibility factor and an output proportional to the mass flow.

6. In the device adapted for determining the mass flow of a gas through a duct, the combination comprising:

(a) means for transmitting sonic waves in both upstream and downstream directions through a section of the duct;

(b) means for detecting the difference in travel time between an upstream travelling wave and a downstream travelling wave; and

(c) means for sensing an adiabatic compressibility factor for the gas, the sensing means including compressor means for compressing a known volume of the gas by a preselected amount and a pressure sensing means connected to the compressor means for sensing the change in the gas pressure.

References Cited UNITED STATES PATENTS 2,911,825 11/1959 Kritz 73-194 2,911,826 11/1959 Kritz 73-194 3,083,569 4/1963 Thomas et al 73l94 3,248,942 5/1966 Cole 73231 3,388,597 6/1968 Bargen et a1. 73-231 CHARLES A. RUEHL, Primary Examiner 

